The present invention is in the field of photochromic glasses and particularly relates to non-silver halide photochromic glasses containing copper halide constituents as the photochromic phase.
Photochromic glasses containing silver halide microcrystals as the photochromic phase are by far the most common of the commercially produced photochromic glasses, being used primarily for optical and ophthalmic applications. However, the relatively high cost of these glasses is disadvantageous for glazing and similar uses where large volumes of glass are required.
Silver-free photochromic glasses containing copper and/or cadmium halides as photochromic constituents have been described by Araujo in U.S. Pat. No. 3,325,299 and by Seward et al. in U.S. Pat. No. 3,954,485. Also, optically clear photochromic glasses containing both copper and cadmium halides, and exhibiting photochromic properties suitable for ophthalmic applications, have been disclosed by Araujo et al. in a commonly assigned U.S. Pat. application Ser. No. 861,129, filed Dec. 16, 1977, now U.S. Pat. No. 4,166,745.
While useful photochromic properties can be developed in glasses containing both copper and cadmium at times and temperatures moderate enough to avoid the introduction of haze or opacity into the glass, the same has not been true of glass compositions containing only copper halides as the photochromic ingredients. Hence the silver-free cadmium-free glass compositions disclosed by Araujo in U.S. Pat. No. 3,325,299 are extremely difficult to heat-treat for photochromic development without introducing haze or opacifying crystal phases into the glass, primarily because of the rather severe heat treatments required to develop photochromic properties therein. This fact is illustrated by the work of A. A. Kuznetsov et al., reported in the Sov. J. Opt. Technol., 45(3), March 1978, pages 163-165. Yet glasses containing only copper halides as the main photochromic constituents offer a substantial advantage in batch cost, an important factor where large volumes of glass would be required, and in addition do not necessitate the handling of toxic cadmium batch materials.
It is therefore a principal object of the present invention to provide glass compositions for photochromic glasses comprising copper halides as the photochromic constituents which can be treated for photochromic development under relatively mild heat treating conditions of time and temperature, and which heretofore have substantially higher utility for the production of optically clear copper halide photochromic glasses than the compositions known to the prior art.
It is a further object of the invention to provide glass compositions useful for producing copper halide-containing glasses treatable for photochromic development at temperatures substantially below the softening point of the glass, so that the surface quality of the glass article made therefrom may remain substantially unaffected by the photochromic development heat treatment.
Other objects and advantages of the invention will become apparent from the following description thereof.